Method of enriching the isotope carbon 13 by means of laser irradiation

ABSTRACT

An enrichment method of carbon 13 by use of the multistage laser irradiation. A mixture of CHClF 2  and Br 2  is used as a start substance. The mixture is irradiated with laser rays emitted from a carbon dioxide gas laser to obtain CBr 2  F 2  enriched with carbon 13. This CBr 2  F 2  enriched with carbon 13 is once more irradiated with laser rays emitted from a carbon dioxide gas laser to produce C 2  Br 2  F 4  which is further enriched with carbon 13. Alternatively, the product CBr 2  F 2  of the first laser irradiation is once more irradiated together with O 2  to produce COF 2  which is further enriched with carbon 13.

BACKGROUND OF THE INVENTION

This invention relates to an enrichment method of carbon 13 by use of laser rays and particularly to an enrichment method of carbon 13 by means of laser isotope separation by way of infrared multiple-photon decomposition.

Natural carbon comprises isotopes of mass-numbers 12 and 13, ratios of which are 98.9% and 1.1%, respectively. Conventional enrichment methods of carbon 13 are based on low temperature distillation of CO, but these methods have such disadvantages that a large amount of poisonous gas should be used, large scale apparatus should be used and manufacturing cost is high. Therefore, it is very significant if carbon 13 is separated safely and cheaply by use of laser irradiation.

We will explain prior art enrichment methods of carbon 13 by the laser irradiation.

Working substance such as CF₃ X (wherein X is Cl, Br or I), or CF₂ HCl is irradiated with laser rays emitted from a CO₂ laser. In the infrared multiple photon decomposition of CF₃ X, the final product is C₂ F₆. In the infrared multiple photon decomposition of CF₂ HCl, the final product is C₂ F₄. The final products are enriched with Carbon 13.

In this method, so long as practically meaningfully high yield is intended, the ratio of carbon 13 after the enrichment is only 80% and does not reach to the ratio obtained in the conventional enrichment methods by way of the low temperature distillation of CO. In the method described in Japanese patent application public disclosue No. 60(1985)-132629, C₂ F₆ is irradiated in presence of Br₂ with pulsed laser rays emitted from a CO₂ laser with adequate wavenumber and fluence. As a result of the photo-dissociation and the subsequent reaction, CF₃ Br is produced which is enriched with carbon 13 to a ratio of 20% to 30%. Next, the product CF₃ Br is separated and once more irradiated with pulsed laser rays emitted from a CO₂ laser under particular conditions to induce an infrared multiple photon decomposition. The product of the second infrared multiple photon decomposition is C₂ F₆, in which the ratio of carbon 13 is increased to 90%.

SUMMARY OF THE INVENTION

Based on knowledge obtained from the above investigations, we invented a new enrichement method of carbon 13 by use of multistage laser irradiation. Namely, the laser irradiation is used plural times in order to gradually increase the concentration of carbon 13 because by use of only once laser irradiation the degree of enrichment does not exceed a limitation.

An object of this invention is to provide a method fulfilling the important requirements from the view point of practical use that the working substance easily causes the multiple photon decomposition, the working substance is largely and cheaply obtainable, the product of the first infrared multiple photon decomposition is directly usable in the second decomposition and the materials obtained during the decomposition process are reusable.

This invention relates to an enrichment method of carbon 13 by use of the multistage laser irradiation. This invention characterized in that a mixture of CHClF₂ and Br₂ is used as a start substance. The mixture is irradiated with laser rays emitted from a CO₂ laser to obtain CBr₂ F₂ enriched with carbon 13. This CBr₂ F₂ enriched with carbon 13 is once more irradiated with laser rays emitted from a CO₂ laser to produce C₂ Br₂ F₄ which is further enriched with carbon 13.

Alternatively, the product CBr₂ F₂ of the first laser irradiation is once more irradiated together with O₂ to produce COF₂ which is further enriched with carbon 13.

FIG. 1 schematically shows the reaction process according to this invention.

When natural CHClF₂ is irradiated with pulsed laser emitted from CO₂ TEA laser, infrared multiple photon decomposition is caused to produce CF₂ and HCl. The laser rays are needed to be mildly focused by a lens in order to induce the decomposition. If the wavenumber of the laser rays is settled at about 1030 to 1050 cm⁻¹ and the fluence is settled below 10 Jcm⁻², the product C₂ F₄ is enriched with ¹³ C. This result is explained by the following reaction mechanism.

    CHClF.sub.2 +nhν→CF.sub.2 +HCl                   (1)

    CF.sub.2 +CF.sub.2 →C.sub.2 F.sub.4                 ( 2)

The reaction (1) represents the process that CHClF₂ absorbs a large number of photons and it causes decomposition. Under the above described irradiation conditions, molcules including ¹³ C are selectively decomposed and CF₂ radicals including a large amount of ¹³ C are generated. According to this coupling reaction of radicals (2), C₂ F₄ enriched with ¹³ C is produced.

In the case that a large amount of Br₂ exsits in the reaction system, the following reaction will happen:

    CF.sub.2 +Br.sub.2 →CBrF.sub.2 +Br                  (3)

    CF.sub.2 +Br.sub.2 →CBr.sub.2 F.sub.2               ( 3')

    CBrF.sub.2 +Br.sub.2 →CBr.sub.2 F.sub.2 +Br         (4)

    CBrF.sub.2 +CBrF.sub.2 →C.sub.2 Br.sub.2 F.sub.4    ( 5)

    Br+Br →Br.sub.2                                     ( 6)

The CF₂ radicals enriched with ¹³ C change to CBrF₂ radicals as the reaction (3) or to CBr₂ F₂ as the reaction (3'), so that the reaction (2) is completely prevented. In consequence of the subsequent reactions (4) and (5), CBr₂ F₂ and C₂ Br₂ F₄ are obtained as products. The relative ratio thereof varies depending upon the laser irradiation conditions and the added amount of Br₂. But the production amount of CBr₂ F₂ is much greater than that of C₂ Br₂ F₄. The production CBr₂ F₂ is highly enriched with ¹³ C.

As a result of the ¹³ C selective infrared multiple photon decomposition of the mixture of natural CHClF₂ and Br₂, (a) CHClF₂ depleted of ¹³ C, (b) CBr₂ F₂ enriched with ¹³ C, (c) C₂ Br₂ F₄ enriched with ¹³ C, (d) HCl, and (e) unreacted Br₂ exist in the reaction system. In order to separate each composition, a low temperature distillation method can be adopted by virtue of those distinct boiling points (CHClF₂ is -40.8° C., CBr₂ F₂ is 24.5° C., C₂ Br₂ F₄ is 47.3° C., HCl is -85° C., Br₂ is 58.8° C.). Alternatively, if the amount to be processed is small, the separation can be caused by a preparative type gaschromatograph having a column packed with silica gel.

As a result, CHClF₂, CBr₂ F₂ and C₂ Br₂ F₄ are almost completely separated.

In the infrared absorption spectrum of natural CBr₂ F₂, a strong absorption band is recognized at 1095 cm⁻¹. This is due to stretching vibration of ¹² C-F bond. The wave number of the absorption band due to stretching vibration of ¹³ C-F bond is lower than that of ¹² C-F bond by 20 to 30 cm⁻¹, that is, 1065 to 1075 cm⁻¹. Therefore, in the infrared multiple photon decomposition of CBr₂ F₂ there is a large isotope effect concerning carbon. When natural CBr₂ F₂ of 5 Torr is irradiated with pulsed laser rays at a wave number of 9P(28), that is, 1039.37 cm⁻¹ and a fluence of 3 J cm⁻² generated by a CO₂ laser, C₂ Br₂ F₄ is produced as a main product according to the following reaction mechanism and the abundance ratio of ¹³ C reaches to 40%.

    CBr.sub.2 F.sub.2 +nhν→CBrF.sub.2 +Br            (8)

    CBrF.sub.2 +CBrF.sub.2 →C.sub.2 Br.sub.2 F.sub.4    ( 9)

    Br+Br→Br.sub.2                                      ( 10)

There is a great selectivity for ¹³ C in the photochemical decomposition process of the reaction (8).

Therefore, if the CBr₂ F₂ products obtained by the first laser irradiation is further irradiated with laser rays emitted from a CO₂ laser, the second stage infrared multiple photon decomposition makes the final product highly enriched with ¹³ C.

On the other hand, when CBr₂ F₂ is irradiated together with O₂, the final product becomes COF₂ which is also enriched with ¹³ C.

According to this invention, the enrichment of carbon 13 is quite easily accomplished by the multistage laser irradiation and carbon 13 can be obtained in high yield. Since the start working substance CHClF₂ is largely and cheaply manufactured and substances obtained during the reaction process are reusable, this invention brings a great advantage when it is industrially used.

CO₂ is adequate for the start substance for synthesising organic compounds labeled by ¹³ C. In the case that the second stage infrared multiple photon decomposition is caused in the mixture of CBr₂ F₂ and O₂ and then the product is treated with water, CO₂ enriched with ¹³ C is directly obtained.

The specific nature of this invention, as well as other objects, uses and advantages thereof, will be clear from the description and the accompanying drawing.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a flowchart illustrating steps to obtain a final product enriched with ¹³ C according to this invention.

EXAMPLE

CO₂ TEA laser was calibrated to oscilate and output laser rays at 9P(22) line of about 4J/pulse. The wavenumber of the laser rays was 1045.02 cm⁻¹. The laser rays were focused by a long focal lens and irradiated therewith the mixture of CHClF₂ of 50 Torr and Br₂ of 10 Torr which was enclosed in a reaction cell of about 3 m in length and 5 lit in volume with windows of NaCl at opposite ends. The number of irradiated pulses was restricted so that about 0.5% of CHClF₂ was decomposed. The product including carbon was mainly CBr₂ F₂ and another product was a small amount of C₂ Br₂ F₄. The irradiated gas was taken into the preparative type gaschromatograph with a column of 17.5 mm in diameter and 3.5 m in lengh with a column packed with silca gel and CBr₂ F₂ was separated. The results of mass spectrometry for the CBr₂ F₂ are as follows:

                  TABLE I                                                          ______________________________________                                         Ion signal intensities of CBrF.sub.2 .sup.+ ion fragment                       under the irradiation conditions that laser rays were focused                  a lens of 1.7 m in focal length and the fluence was 7 J cm.sup.-2.                        Relative intensities of                                             m/e        ion signals                                                         ______________________________________                                         129        1.00                                                                130        0.43                                                                131        0.98                                                                132        0.42                                                                .sup.13 C/(.sup.12 C + .sup.13 C) = 30%                                        ______________________________________                                    

                  TABLE II                                                         ______________________________________                                         Ion signal intensities of CBrF.sub.2.sup.+ ion fragment                        under the irradiation conditions that the laser rays were                      focused by a lens of 3.0 m in focal length and the fluence                     2.2 J cm.sup.-2.                                                               ______________________________________                                                    Relative intensities of                                             m/e        ion signals                                                         ______________________________________                                         129        1.00                                                                130        1.22                                                                131        0.98                                                                132        1.20                                                                .sup.13 C/(.sup.12 C + .sup.13 C) = 55%                                        ______________________________________                                    

In the mass spectroscopic analysis of CBr₂ F₂, ion signals of CBrF₂ ⁺ are most strong. The isotope species thereof are ¹² C⁷⁹ Br¹⁹ F¹⁹ F⁺ (m/e=129), ¹³ C⁷⁹ Br¹⁹ F¹⁹ F⁺ (m/e=130), ¹² C⁸¹ Br¹⁹ F¹⁹ F⁺ (m/e=131) and ¹³ C⁸¹ Br¹⁹ F¹⁹ F⁺ (m/e=132). In the case that the laser rays were focused by a lens of 3 m in focal length, the ratio of ¹³ C in the CBr₂ F₂ molecule was reached to 55%.

Next, CBr₂ F₂ (the ratio of ¹³ C atoms was 30%), which was produced in the first stage infrared multiple photon decomposition and separated, was taken into a reaction cell and irradiated with pulsed laser rays at 9P(28) line, that is, 1039.37 cm⁻¹ emitted from a CO₂ laser apparatus. The fluence of the laser rays was 3.3 J cm⁻² and the the number of the irradiated pulses was 100. The pressure of CBr₂ F₂ was about 10 Torr. The product was C₂ Br₂ F₄. The ratios of ¹² C and ¹³ C atoms in the C₂ Br₂ F₄ molecules, which were measured by means of gas chromatography(GC) and massspectrometry(MS), were shown in Table III.

                  TABLE III                                                        ______________________________________                                                             Relative intensities of                                    Ion species  m/e    ion signals                                                ______________________________________                                         .sup.12 CF.sup.+                                                                            31     0.05                                                       .sup.13 CF.sup.+                                                                            32     1.00                                                       .sup.13 C/(.sup.12 C + .sup.13 C) = 95%                                        ______________________________________                                    

In the gas chromatography, Gaskuropack 55 with a column of 3 mm×6 m was used at 150° C. The ion, which was aimed during the measurement by the mass spectrometer, was CF⁺. It was found that the ratio of ¹³ C was increased to 95%.

On the other hand, O₂ at 10 Torr was added to CBr₂ F₂ (the ratio of ¹³ C atoms was 30%) at 10 Torr. The mixed gas was irradiated with 300 laser pulses under the same irradiation conditions that the wavenumber of the laser rays was 1039.37 cm⁻¹ and the fluence was 3.3 J cm⁻². In the case of O₂ existence, the product was COF₂. When H₂ O reacted with this product, it rapidly changed to CO₂. The ratios of ¹³ C and ¹² C atoms in the CO₂ molecules, which were measured by GC and MS, were shown in Table IV.

                  TABLE IV                                                         ______________________________________                                                             Relative intensities of                                    Ion species  m/e    ion signals                                                ______________________________________                                         .sup.12 CO.sub.2 .sup.+                                                                     44     0.10                                                       .sup.13 CO.sub.2 .sup.+                                                                     45     1.00                                                       .sup.13 C/(.sup.12 C + .sup.13 C) = 91%                                        ______________________________________                                    

From this result, it was found that the ratio of ¹³ C was increased to 91%. 

What is claimed is:
 1. A method of enriching the isotope carbon 13 by means of laser irradiation, comprisingirradiating a mixture of CHClF₂ and Br₂ with laser rays emitted from a carbon dioxide gas laser to produce CBr₂ F₂, and irradiating the produced CBr₂ F₂ with laser rays emitted from a carbon dioxide gas laser.
 2. A method of enriching the isotope carbon 13 by means of laser irradiation, comprisingirradiating a mixture of CHClF₂ and Br₂ with laser rays emitted from a carbon dioxide gas laser to produce CBr₂ F₂, and irradiating a mixture of the produced CBr₂ F₂ and O₂ with laser rays emitted from a carbon dioxide gas laser. 